1. Field of the Invention
A remotely conrolled variable capacitor has many uses in the electronic circuitry and a remotely controlled unit is needed in many places where direct mechanical tuning is impossible. For example, in the matching of the impedance of a whip antenna to the output of a transmitter at various frequencies, particularly in pack sets, a variable control element, like an electrically tunable capacitor or inductor, is necessary. However, for use in a portable set, this element must be small, lightweight and consume very little battery power. It should also have linear characteristics so as to be able to handle the radio frequency output power of the transmitter without additional distortion and, of course, it must be reasonably inexpensive.
2. Description of the Prior Art
One type of remotely controlled variable capacitor is the Varactor Diode which is a semiconductor junction device whose capacity varies with the back biased voltage applied. This device is nonlinear and causes the transmitter current to create additional spurious signals, especially harmonics, due to the nonlinearity.
Another type of remotely controlled variable capacitor is the Ferroelectric Barium Titanate Capacitor, wherein the capacity varies with the DC voltage applied. However, their narrow temperature range prohibits use in rugged, pack set radios.
Impedance matching may also be accomplished with variable inductors or saturable reactors such as "Vari-L" or "increductor" wherein the inductance of a coil is changed by magnetizing its core with a magnetic field to vary its permeability. This magnetic field is normally created by an electromagnet which requires direct current power and is comparatively heavy. A permanent magnet may also be used but its field must still be varied by a secondary electromagnetic field controlled by a direct current. In any case, the device is comparatively big and cumbersome and is not optimum for use with portable pack set radios.